Transaxle and hydrostatic transmission assemblies are known in the art. Generally, a hydrostatic transmission assembly includes a rotating hydraulic pump and a rotating hydraulic motor. The hydraulic pump usually is a piston-type pump including a plurality of reciprocating pistons which are in fluid communication through hydraulic porting with the hydraulic motor. Rotation of the hydraulic pump against a moveable swash plate creates an axial motion of the pump pistons that forces hydraulic fluid through the hydraulic porting to the hydraulic motor to drive the motor. The rotation of the hydraulic motor may be used to drive a wheel axle of a riding lawn mower, small tractor and the like.
The position of the swash plate with respect to the hydraulic pump pistons can be changed to vary the speed and change the direction of rotation of the hydraulic motor and thus the speed and direction of rotation of the vehicle axle. The orientation with which the swash plate addresses the hydraulic pump pistons can be changed to control whether the hydraulic motor rotates in the forward direction or in the reverse direction. Additionally, the angle at which the swash plate addresses the hydraulic pump pistons can be changed to increase or decrease the amount of hydraulic fluid that is forced from the hydraulic pump to the hydraulic motor to change the speed at which the hydraulic motor rotates.
The position of the moveable swash plate typically is varied by rotating a trunnion arm that is coupled to the swash plate. The trunnion arm projects from the pump housing for coupling to a speed change lever or a speed change pedal of the vehicle through a cable, lever or other linkage. In this manner, movement of the speed change lever/pedal results in rotational movement of the trunnion arm to change the position of the swash plate to thereby control the speed and direction of the vehicle.
Return to neutral mechanisms heretofore have been used to cause the swash plate of the pump to assume a neutral position even after having been shifted to a forward and/or reverse position. The return to neutral mechanism is normally implemented as an integral part of the vehicle linkage.
In U.S. Pat. No. 6,782,797 there is disclosed a return to neutral mechanism that is adapted to be mounted to the casing of a hydraulic pump or hydrostatic transaxle. The mechanism is integrated into a speed adjusting mechanism that includes a return arm adapted to be mounted to the casing in a fixed position indicative of a neutral position of the trunnion arm coupled to the swash plate of the pump. Additionally, the speed adjusting mechanism includes a control arm that is mounted to and moves the trunnion arm. A pair of scissor return arms are provided that are adapted to move the control arm in cooperation with the return arm for the purpose of moving the trunnion arm to the neutral position. To provide mounting flexibility, the control arm is adapted to be mounted to the trunnion arm in any one of a plurality of different positions and the return arm is capable of being mounted to the casing in a corresponding position such that the return arm can be aligned with the control arm to establish the neutral position.